fix livelog somewhere in taskrun disposing...

[jonasfj]

Following: https://bug1405767.bmoattachments.org/attachment.cgi?id=8945195
credits to bc

I suspect this is somewhere in the taskrun dispose phase..specifically, in:
https://github.com/taskcluster/taskcluster-worker/blob/master/runtime/taskcontext.go#L125

definite proof is still needed, so we should:

1. add some more debug statements
2. upgrade the stream library... and check the commit logs to see if there was a fix for something like this.